Purification method for (meth)acrylic acid

ABSTRACT

(Meth)acrylic acid is purified by crystallization by a process in which the (meth)acrylic acid is purified by a combination of at least two different dynamic crystallization processes, in particular a suspension crystallization and a layer crystallization.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a purification process which issuitable both for methacrylic acid and for acrylic acid. Below, the term(meth)acrylic acid means methacrylic acid or acrylic acid.

2. Description of the Background

Acrylic acid is a key chemical. Owing to its very reactive double bondand the acid function, it is particularly suitable as a monomer for thepreparation of polymers. The greater part of the amount of acrylic acidmonomers produced is esterified prior to polymerization—to give, forexample, adhesives, dispersions or finishes. Only the smaller part ofthe acrylic acid monomers produced is polymerized directly—to give, forexample, superabsorbers. While monomers of high purity are generallyrequired in the direct polymerization of acrylic acid, the requirementsrelating to the purity of the acrylic acid are not so high when it isesterified prior to polymerization.

It is generally known that acrylic acid can be prepared by gas-phaseoxidation of propene with molecular oxygen under heterogeneous catalysisover solid catalysts at from 200 to 400° C., in two stages via acrolein.Here, oxidic multicomponent catalysts, for example based on oxides ofthe elements molybdenum, chromium, vanadium or tellurium, are used.

Several processes have been proposed for working up the gas mixtureobtained in the catalytic gas-phase oxidation.

WO-A-9 801 415 discloses a process for the preparation of (meth)acrylicacid in which the gas mixture obtained by catalytic gas-phase oxidationis condensed and (meth)acrylic acid can be crystallized from theresulting aqueous solution without addition of assistants.

EP-B-0 616 998 describes a process for purifying acrylic acid by meansof fractional crystallization, the acrylic acid being purified by acombination of dynamic and static crystallization in a plurality ofstages by means of crystallization/melting cycles, and the residue ofthe dynamic crystallization being further concentrated by the staticcrystallization. The dynamic crystallization described is either afalling-film layer crystallization or a suspension crystallization.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpurifying (meth)acrylic acid which permits particularly economicalworking-up for a wide concentration range of the acids crystallizing anda wide yield range.

We have found that this object is achieved by the combination of atleast two different dynamic crystallization processes.

The present invention therefore relates to a process for purifying(meth)acrylic acid by crystallization, wherein the (meth)acrylic acid ispurified by a combination of at least two different dynamiccrystallization processes. Preferred embodiments of the invention aredescribed in the following description, the Figures, the Example and thesubclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures:

FIGS. 1A and B show preferred processes comprising suspension and layercrystallization;

FIGS. 2A and B show preferred processes comprising additional staticcrystallization and/or distillation;

FIG. 3A shows a further preferred process comprising two layercrystallization stages, a suspension crystallization stage and staticcrystallization and/or distillation;

FIG. 3B shows a further preferred process comprising two layercrystallization stages, two suspension crystallization stages and adistillation; and

FIG. 4 shows a further preferred embodiment comprising a suspensioncrystallization stage and five layer crystallization stages.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Suitable mixtures from which the starting mixture for the presentprocess can be obtained are preferably prepared by catalytic gas-phaseoxidation of C₃- or C₄-alkanes, C₃- or C₄-alkenes, C₃- or C₄-alkanolsand/or C₃- or C₄-alkanals and/or precursors thereof. The mixture isparticularly advantageously prepared by catalytic gas-phase oxidation ofpropene, acrolein, tert-butanol, isobutene, isobutane, isobutyraldehyde,methacrolein, isobutyric acid or methyl tert-butyl ether. It is possibleto use all precursors of said compounds, in which the actual C₃/C₄starting compound is formed only as an intermediate during the gas-phaseoxidation.

Methyl tert-butyl ether or isobutyric acid may be mentioned by way ofexample for the preparation of methacrylic acid.

The catalytic gas-phase oxidation of propene and/or acrolein to acrylicacid with molecular oxygen by known processes, for example according toDE-A-196 24 31, is particularly advantageous. Here, temperatures of from200 to 450° C. and, if required, superatmospheric pressure arepreferably employed. Preferably used heterogeneous catalysts are oxidicmulticomponent catalysts based on oxides of molybdenum, bismuth and ironin the 1st stage (oxidation of propene to acrolein) and those based onthe oxides of molybdenum and vanadium in the second stage (oxidation ofacrolein to acrylic acid). If propane is used as a starting material, itcan be reacted to give a propene/propane mixture by catalyticoxydehydrogenation, as described in U.S. Pat. No. 5,510,558, byhomogeneous oxydehydrogenation, corresponding to CN-A-1 105 352, or bycatalytic dehydrogenation, as described in EP-A-0 253 409. When apropene/propane mixture is used, propane acts as a diluent gas. Othersuitable propene/propane mixtures are refinery propene (70% of propeneand 30% of propane), or cracker propene (95% of propene and 5% ofpropane). In principle, propene/propane mixtures such as theabovementioned ones can be oxidized with oxygen or air or a mixture ofoxygen and nitrogen of any composition to give acrolein and acrylicacid.

The reaction of propene to give acrylic acid is highly exothermic. Thereaction gas, which advantageously contains an inert diluent gas, e.g.atmospheric nitrogen, or one or more saturated C₁-C₆-hydrocarbons, inparticular methane and/or propane, and/or steam, in addition to startingmaterials and products, can therefore absorb only a small part of theheat of reaction. Although the type of reactors used is not subject toany restriction per se, tube-bundle heat exchangers filled with theoxidation catalyst are generally used since in these the predominantpart of the heat liberated during the reaction can be removed byconvection and radiation to the cooled tube walls.

In the catalytic gas-phase oxidation, it is not pure acrylic acid whichis obtained but a gaseous mixture which contains essentially unconvertedacrolein and/or propene, steam, carbon monoxide, carbon dioxide,nitrogen, propane, oxygen, acetic acid, propionic acid, formaldehyde,further aldehydes and maleic anhydride as secondary components inaddition to the acrylic acid. Usually, the reaction product mixturecontains from 1 to 30% by weight of acrylic acid, from 0.05 to 1% byweight of propene and from 0.05 to 1% by weight of acrolein, from 0.05to 10% by weight of oxygen, from 0.05 to 2% by weight of acetic acid,from 0.01 to 2% by weight of propionic acid, from 0.05 to 1% by weightof formaldehyde, from 0.05 to 2% by weight of aldehydes, from 0.01 to0.5% by weight of maleic anhydride and from 20 to 98, preferably from 50to 98%, by weight of inert diluent gases, based in each case on thetotal reaction mixture. In particular, saturated C₁-C₆-hydrocarbons,such as from 0 to 90% by weight of methane and/or propane, as well asfrom 1 to 30% by weight of steam, from 0.05 to 15% by weight of oxidesof carbon and from 0 to 90% by weight of nitrogen, based in each case on100% by weight of diluent gas, are present as inert diluent gases.

The methacrylic acid can be prepared analogously to acrylic acid bycatalytic gas-phase oxidation of C₄ starting compounds with molecularoxygen. The methacrylic acid is particularly advantageously obtainableby catalytic gas-phase oxidation of isobutene, isobutane, tert-butanol,isobutyraldehyde, methacrolein or methyl tert-butyl ether. The catalystsused are likewise catalysts comprising mixed oxides of transition metals(e.g. Mo, V, W and/or Fe). Particularly suitable processes are those inwhich the preparation is carried out starting from methacrolein,particularly when the methacrolein is produced by gas-phase catalyticoxidation of tert-butanol, isobutane or isobutene or by reaction offormaldehyde with propionaldehyde according to EP-B-0 092 097. Thus, itis also possible to prepare methacrylic acid in two stages by (1)condensation of propionaldehyde with formaldehyde (in the presence of asecondary amine as a catalyst) to give methacrolein and (2) subsequentoxidation of the methacrolein to methacrylic acid.

As in the preparation of acrylic acid, it is not pure methacrylic acidwhich is obtained but a gaseous mixture which can essentially containunconverted methacrolein and/or steam, carbon monoxide, carbon dioxide,nitrogen, oxygen, acetic acid, propionic acid, further aldehydes andmaleic anhydride as secondary components in addition to the methacrylicacid.

For the preparation of the starting mixture for the present process, thereaction product mixture described above can be subjected to acondensation, in particular a partial or total condensation, a solutionbeing obtained. The condensation can be carried out in one or morestages by conventional processes, the type of condensation not beingsubject to any particular restriction. Advantageously, the condensationis carried out using a direct condenser, condensate produced beingbrought into contact with the hot gaseous reaction product. Suitableapparatuses for the condensation are in particular spray scrubbers,venturi scrubbers, bubble columns or apparatuses having sprayedsurfaces.

The (meth)acrylic acid-containing starting mixture for the novelpurification process is not subject to any particular restrictions.Expediently, it contains from 75 to 99.9, preferably from 90 to 99.7,particularly preferably from 95 to 99.5, % by weight, based in each caseon 100% by weight of starting mixture, of (meth)acrylic acid. Theremainder comprises water and further impurities, such as acids,aldehydes, in particular acrolein, acetic acid, propionic acid,diacrylic acid, maleic acid (anhydride) and the polymerization inhibitorphenothiazine.

According to the invention, the starting mixture is subjected to acombination of at least two different dynamic crystallization processes,i.e. at least two crystallization stages. Preferably, the dynamiccrystallization stages combined with one another are carried out as afractional crystallization. In fractional crystallization, all stagesabove the feed of the starting mixture, i.e. in the direction of purermixtures, are usually referred to as purification stages and all otherstages, i.e. below the feed of the starting mixture, are usuallyreferred to as stripping stages. Expediently, multistage crystallizationprocesses are operated according to the countercurrent principle, wherethe crystals of each stage, after separation from the mother liquor, arefed to the respective stage with the next highest purity while thecrystallization residue, i.e. the mother liquor, is fed to therespective stage with the next lowest purity. According to the purity ofthe respective crystals, the crystallization stages are usually referredto as higher or lower crystallization stages. Accordingly, the strippingstage which produces the crystals or the mother liquor with the lowestpurity is referred to as the lowest crystallization stage and thepurification stage with the highest purity is referred to as the higheststage.

According to the invention, the number of crystallization stages used isnot subject in principle to any restrictions. In particular, it isdetermined by the composition of the starting mixture, the desiredpurity of the end product and the desired yield. Advantageously, thefractional crystallization is carried out using at least one strippingstage and at least one purification stage.

The dynamic crystallization processes used according to the inventionare in principle any crystallization process which is carried out withforced movement of the mother liquor. According to the invention, atleast two different dynamic crystallization processes are used, the useof two different dynamic crystallization processes being particularlypreferred. The type of dynamic crystallization processes used is notsubject to any particular restrictions. Suitable processes aresuspension crystallization, falling-film layer crystallization, layercrystallization of the type with full flow through a tube or layercrystallization on moving cooling surfaces (cooling belt, chill roll), acombination of suspension crystallization and falling-film layercrystallization being preferably used.

The novel process is not restricted with regard to the procedure for thedynamic crystallization processes used. These may be continuous orbatchwise. Both suspension crystallization and layer crystallization canbe operated continuously or batchwise. Preferably, the suspensioncrystallization and a layer crystallization are carried out continuouslyon moving cooling surfaces while the falling-film layer crystallizationand the layer crystallization of the type with full flow through a tubeare operated in particular batchwise. In a further preferred embodiment,a suspension crystallization is carried out below the layercrystallization. Particularly preferably, the suspension crystallizationis used in the stripping section, in particular in all stripping stages,while the layer crystallization is preferably used in the purificationsection, in particular in all purification stages.

The removal of heat in the dynamic crystallization processes canpreferably be effected by cooling apparatus walls or by partialevaporation of the crystallizing solution under reduced pressure.Particularly preferably, the heat is removed by indirect cooling bymeans of heat exchanger surfaces. All mixtures suitable for thispurpose, in particular water/methanol or water/glycol mixtures, may beused as heat-transfer media.

Advantageously, the temperature of the mother liquor during the dynamiccrystallization is from −30 to +15° C., in particular from −10 to +15°C., particularly preferably from −5 to +14° C.

The suspension crystallization is a crystallization process in which,from a generally solid-free, liquid multicomponent system, as startingmaterial, which is present in solution or as a melt, single crystals areformed by heat removal in the mass of the starting material. The crystalsuspension containing the mother liquor and the solid phase consistingof dispersed single crystals has to be agitated during the suspensioncrystallization process, for which purpose circulation by pumping orstirring is particularly suitable. Adhesion of crystals to surfaces isnot necessary here and is even undesirable. Since the crystal suspensionhas to be agitated, the suspension crystallization is considered to be adynamic crystallization process.

In the suspension crystallization by indirect cooling, the heat isremoved by means of scraped-surface heat exchangers which are connectedto a stirred kettle or to a container without a stirrer. The circulationof the crystal suspension is ensured here by means of a pump. It is alsopossible to remove the heat via the wall of a stirred kettle having astirrer passing close to the wall. A further preferred embodiment in thecase of the suspension crystallization is the use of cooling diskcrystallizers, as produced, for example, by GMF (Gouda in theNetherlands). In a further suitable variant of the suspensioncrystallization by cooling, the heat is removed by means of conventionalheat exchangers (preferably tube-bundle or plate-type heat exchangers).In contrast to scraped-surface heat exchangers, stirred kettles havingstirrers passing close to the wall or cooling disks, these apparatuseshave no means for avoiding crystal layers on the heat-transfer surfaces.If, during operation, a state in which the heat transfer resistanceassumes too high a value owing to incrustation is reached, the operationis switched to a second apparatus. During the operating time of thesecond apparatus, the first apparatus is regenerated (preferably bymelting off the crystal layer or flushing the apparatus with unsaturatedsolution). If too high a heat transfer resistance is reached in thesecond apparatus, the operation is switched back to the first apparatus,etc. This variant can also be operated with more than two apparatuses orcyclically. Moreover, the crystallization can be effected byconventional evaporation of the solution under reduced pressure.

All known solid-liquid separation methods are suitable for separatingthe solid-liquid mixture obtained after the dynamic crystallization.Preferably, the crystals are separated from the mother liquor byfiltration, sedimentation and/or centrifuging. However, it is alsopossible to remove the mother liquor from then preferably stationarycrystals, for example by allowing it to run off. Advantageously, thefiltration, sedimentation or centrifuging is preceded by preliminarythickening of the suspension, for example by hydrocyclones. All knowncentrifuges which operate batchwise or continuously are suitable for thecentrifuging. Single-stage or multistage reciprocating-conveyorcentrifuges are particularly advantageously used. Scroll-conveyorcentrifuges or helical-conveyor centrifuges (decanters) are alsosuitable. Filtration is advantageously carried out by means of suctionfilters, which are operated continuously or batchwise, with or without astirrer, or by means of belt filters. In general, the filtration can becarried out under superatmospheric or reduced pressure. Further processsteps for increasing the purity of the crystals or of the crystal cakecan be provided during and/or after the solid-liquid separation. In aparticularly advantageous embodiment of the invention, the separation ofthe crystals from the mother liquor is followed by single-stage ormultistage washing and/or sweating of the crystals or of the crystalcake. The wash liquid used is not subject to any restriction here.Advantageously, however, washing is effected with pure product, i.e.with a liquid which contains the acid whose purity is higher than thatof the mother liquor. Washing with water is also possible. Washing canbe carried out in apparatuses customary for this purpose, such as washcolumns, in which the removal of the mother liquor and the washing arecarried out in one apparatus, in centrifuges, which can be operated inone or more stages, or in suction filters or belt filters. The washingcan be carried out on centrifuges or belt filters in one or more stages,it being possible to transport the wash liquid counter currently to thecrystal cake. Sweating for increasing the purity of the crystals, whichinvolves local melting of contaminated regions, can also be provided. Inthe suspension crystallization, it is particularly preferable to carryout sweating on centrifuges or belt filters, but carrying out acombination of washing and sweating in one apparatus may also besuitable.

In a particularly suitable manner, the wash liquid used for the crystalsof a given crystallization stage is the feed to the same crystallizationstage. Preferably, the mass ratio of wash liquid to crystals ispreferably set at from 0.1 to 1, particularly preferably from 0.2 to0.6, kg of wash liquid to kg of crystals.

There are no restrictions with regard to carrying out the dynamic layercrystallization, preferably a falling-film layer crystallization or alayer crystallization of the type with full flow through a tube. Thedynamic layer crystallization on stationary cooling surfaces can becarried out as follows. The crystals of the acid are applied to thecooling surface by bringing the cooling surface into contact with aliquid mixture which contains the acid to be purified and by forming thecorresponding crystals by cooling the cooling surface. For the formationof the crystals, the cooling surface is preferably cooled to atemperature range from the dissolution temperature of the respectiveliquid mixture to 60 K below this, preferably to 40 K below this. Onreaching the desired crystal mass, the cooling process is terminated.Thereafter, the uncrystallized residual liquid depleted with respect tothe desired acid can be taken off and thus removed from the coolingsurfaces or the crystals formed. The removal of the residual liquid canbe effected by simply allowing it to run off or by pumping it away.

This can be followed by a wash step and/or sweating step. During thewashing, the crystals grown on the cooling surfaces are brought intocontact with a wash liquid and are separated again from the latter.Consequently, the residual liquid remaining on the crystals is exchangedfor the preferably purer wash liquid. Particularly in the case ofrelatively long residence time of the wash liquid on the crystals,exchange of impurities between the purer wash liquid and less pureregions of the crystals by diffusion is also effected. A preferably usedwash liquid is fresh liquid mixture, which contains the acid to bepurified, or pure melt of the acid. During sweating, the temperature ofthe crystals on the cooling surface is increased, after removal of theresidual liquid, to a value which is from the freezing point of theresidual liquid depleted with respect to the desired acid to the meltingpoint of the pure acid. The sweating is advantageous particularly whenthe crystals of the acid are present not as a compact crystal layer butas a porous, inclusion-rich heap. Thereafter, the crystals can beliquefied by heating and the resulting liquid enriched with desired acidcan be removed, which can once again be effected, for example, by simplyallowing it to run off or pumping it away. The liquefaction of thecrystals is preferably effected in a temperature range from the meltingpoint of the respective acid to 40 K above this, in particular to 20 Kabove this.

In a particular embodiment of the dynamic layer crystallization, liquidenriched with acid and remaining on the cooling surfaces as a residualfilm after the melting is partially or completely frozen to give seedcrystals on the cooling surface, after which the crystallization iscarried out again. The freezing to give seed crystals can also becarried out by applying seed crystals to the cooling surface prior tothe crystallization, by a procedure in which the cooling surface isbrought into contact, in a separate step, with a melt, solution orsuspension of the acid which is purer compared with the liquid mixtureto be separated, and subsequently separated therefrom, and correspondingseed crystals are then formed by cooling. Here too, the residual filmremaining on the cooling surfaces is partially or completely frozen bydecreasing the temperature at the surfaces.

The cooling surfaces which can be used in the dynamic layercrystallization are not subject to any restriction and may be of anydesired shape. One or more cooling surfaces may be used. Preferably,cylindrical cooling surfaces, e.g. pipes, or flat cooling surfaces areused. Here, either the cooling surfaces can be completely immersed inthe liquid from which the desired acid is to be purified or a tricklefilm of this liquid may flow over said cooling surfaces, e.g. a pipethrough which there is full flow or through or over which a trickle filmflows. The cooling surfaces may also be parts of a heat exchanger whichare provided with a feed and a discharge.

Thus, a particular embodiment of the dynamic layer crystallization maybe summarized as a batchwise process having the following sequence:

-   1. Cooling surface(s) and the liquid mixture which contains the acid    to be purified are brought into contact;-   2. the cooling surfaces are cooled so that crystals of the acid grow    on the surfaces;-   3. the cooling process is terminated on reaching a desired crystal    mass;-   4. the uncrystallized residual liquid having a lower content of    desired acid is removed from the cooling surfaces or the crystals    formed;-   5. the crystals present on the cooling surfaces are melted off by    increasing the temperature at the surfaces and the resulting liquid    enriched in acid is removed from the surfaces; and-   6. liquid remaining as a residual film on the surfaces and having a    higher acid content can be partially or completely frozen as seed    crystals for the next crystallization cycle by decreasing the    temperature at the surfaces.

The dynamic layer crystallization can be used for both crystallizationfrom the melt and that from solution. It is particularly suitable forcrystallization from the melt.

Falling-film layer crystallization can be carried out, for example, asdescribed in EP-B-0 616 998.

In a preferred embodiment of the novel purification process, thecrystallization is combined with a static crystallization and/or adistillation stage. Here, preferably at least a part of the motherliquor from at least one stripping stage of the crystallization isfurther purified in a static crystallization and/or distillation, inparticular the mother liquor from the lowest dynamic crystallizationstage, preferably a suspension crystallization, being used. There are norestrictions with regard to carrying out the static crystallization; forexample, it is possible to proceed as described in EP-B-0 616 998. Whena distillation stage is used, preferably a part of the resultingdistillation residue is removed and the top product formed is fed to oneor more stripping stages of the crystallization which are below thestripping stage from which the mother liquor for the feed to thedistillation stage was removed. That part of the mother liquor from theat least one stripping stage which is fed to the distillation stage ispreferably from 5 to 100, in particular from 50 to 100, particularlypreferably from 90 to 100, % by weight of the respective mother liquor.

It is possible in principle to use any distillation column for thedistillation. The distillation, distillation stage or distillation stepcan be carried out in one or more stages. In a multistage or fractionaldistillation (also referred to as rectification), which isadvantageously carried out in rectification columns, a column havingsieve trays, e.g. dual-flow trays or crossflow sieve trays of metal, isused. The distillation can also be carried out by means of an evaporatorand a downstream condenser. Here, thin-film evaporators in the form ofdownflow evaporators or thin-film evaporators having rotating wipers areparticularly preferred. The condensers used are conventional condensers,these not being subject to any restrictions. Injection condensers areparticularly preferred. In a one-stage distillation, a simpleevaporator, for example a still, and a conventional condenser areexpediently used.

The invention is explained in more detail with reference to drawingswhich show preferred embodiments of the invention. Here, identicalreference symbols or reference numerals have the same meanings.

FIG. 1A shows an embodiment of crystallization which comprises asuspension crystallization S and a layer crystallization F, both ofwhich can be carried out in one or more stages and continuously orbatchwise. Here, the suspension crystallization S is arranged below thelayer crystallization F. The reference numerals 1, 2 and 3 denotepossible feeds for the starting mixture to be purified. The residue 4 isremoved from the suspension crystallization S while the pure product 5comprising the desired acid is removed from the layer crystallization F.FIG. 1B differs from FIG. 1A in that the layer crystallization isarranged below the suspension crystallization. The reference numeral 6denotes a possible partial or complete removal of the mother liquor orof the residue of the upper dynamic crystallization.

FIG. 2A and FIG. 2B show the embodiments of FIGS. 1A and 1B,respectively, which are supplemented by a static crystallization SCand/or a distillation stage D.

In this case, the residue 4 is removed from the static crystallizationsection and/or from the distillation section.

FIG. 3A shows a preferred arrangement of the novel process comprising aone-stage continuous suspension crystallization S1 as stripping stageand two batchwise falling-film crystallization stages F1 and F2 aspurification stages. The residue of the suspension crystallization S1 isfed to a static crystallization SC and/or to a distillation stage D.

FIG. 3B shows the same combination of the dynamic crystallization stagesS1, F1 and F2 as FIG. 3A, the residue of stage S1 being fed to acombination of distillation and a further suspension crystallizationstage S2, and preferably the suspension crystallization stage S2 beingarranged below the distillation stage D.

FIG. 4 shows, as an embodiment of the invention, the fractionalcrystallization of an acrylic acid mixture, a total of 6 crystallizationstages being used. Stage S1 is a one-stage continuous suspensioncrystallization, and all further stages are batchwise falling-film layercrystallization stages, as described, for example, in EP-B-0 616 998.The falling-film stages F1 to F3 are, like stage S1, stripping stages ofthe crystallization and stages F4 and F5 are purification stages of thecrystallization.

By combining at least two different dynamic crystallization processes,the novel process makes it possible to obtain a pure acid in high yieldin a very economical manner for a wide concentration range of startingacid. By means of the novel, suitable combination of dynamic processes,expensive crystallization stages can be omitted and the complexity ofcrystallization can be reduced. Particularly in comparison with a staticcrystallization, the dynamic crystallization processes have theadvantage of a shorter residence time and a better purification effectper crystallization stage. In economic terms, the separation result isparticularly good when suspension crystallization is arranged below thefalling-film layer crystallization.

The novel process is furthermore explained in more detail with referenceto the following Example, which constitutes a preferred embodiment ofthe invention.

EXAMPLE

A stream according to the starting composition shown in Table 1 wassubjected to a six-stage fractional crystallization using an arrangementas shown in FIG. 4. After passing through the purification stages F4 andF5, a pure product having the composition shown in Table 1 was obtained.After passing through the stripping stages F3 to S1, a residue havingthe composition shown in Table 1 was obtained.

TABLE 1 Starting mixture Pure product Residue (stream 3) (stream 5)(stream 4) Acrylic acid 99.45% by wt. 99.97% by wt. 52.96% by wt. Aceticacid 960 ppm 147 ppm 7.22% by wt. Propionic acid 330 ppm 88 ppm 2.15% bywt. Diacrylic acid 3100 ppm 47 ppm 27.46% by wt. Water 190 ppm 23 ppm1.67% by wt. Phenothiazine 290 ppm <1 ppm 2.57% by wt. Furan-II 220 ppm<1 ppm 1.95% by wt. aldehyde Others 410 ppm <5 ppm 4.02% by wt.

The feed to the suspension crystallization stage S1 was the motherliquor arriving from stage F1 and having the composition shown in Table2. The crystals obtained in stage S1 had the composition shown in Table2 and were fed to stage F1. The mother liquor obtained in stage S1 afterthe crystals had been separated off was partly removed as residue(stream 4) and had the residue composition shown in Table 1. That partof the mother liquor which was not removed was recycled to thecrystallization. The mass ratio of removed to recycled mother liquor was1:8. The suspension crystallization stage S1 was carried out in astirred kettle having a stirrer passing close to the wall. Thecrystallization temperature was −8° C. The residence time of thesuspension crystallization was 4 hours. The crystals produced wereseparated off on a screen centrifuge with a residence time of 1 minuteon the centrifuge (basket diameter 200 mm, 2000 revolutions/minute). Thefilter cake was not washed.

TABLE 2 Feed from F1 to S1 Crystals in S1 Acrylic acid 85.96% by weight93.45% by weight Acetic acid 2.92% by weight 1.95% by weight Propionicacid 1.05% by weight 0.8% by weight Diacrylic acid 7.32% by weight 2.75%by weight Water 0.47% by weight 0.2% by weight Phenothiazine 0.68% byweight 0.26% by weight Furan-II aldehyde 0.52% by weight 0.19% by weightOthers 1.08% by weight 0.4% by weight

The acrylic acid yield achieved using the fractional crystallization was99.4%.

The Example shows that, by means of the combination of two differentdynamic crystallization processes, i.e. a suspension crystallizationwith falling-film layer crystallization, purification of acrylic acid ispossible with high product purity and in particular with high yield ofthe purification process. The concentration of impurities in the residue(=high yield) is possible in the lowest stage even without washing orsweating of the crystals.

Compared with a residence time of 4 hours in the lowest crystallizationstage in the process of the invention, the residence time in thecrystallizer in each stage of the static crystallization duringcrystallization and sweating in the prior art according to EP-A-0 616998 is from 8 to 9 hours. Thus, according to the invention, theresidence time and, by omitting a stripping stage, the complexity ofcrystallization are reduced.

1. A process for purifying (meth)acrylic acid by crystallization, whichcomprises: purifying a (meth)acrylic acid mixture by a combination of atleast two different types of dynamic crystallization processes.
 2. Aprocess as claimed in claim 1, wherein the overall crystallizationprocess occurs by fractional crystallization.
 3. A process as claimed inclaim 1, wherein the purifying occurs by employing two different dynamiccrystallization processes.
 4. A process as claimed in claim 1, whereinthe overall crystallization process occurs by using at least onestripping stage and at least one purification stage.
 5. A process asclaimed in claim 1, wherein a starting mixture comprises from 75 to99.9% by weight, based on 100% by weight of starting mixture, of(meth)acrylic acid.
 6. A process as claimed in claim 1, wherein saidcombination of at least two different types of dynamic crystallizationprocesses is combined with at least one method selected from staticcrystallization and a distillation.
 7. A process for purifying(meth)acrylic acid by crystallization, which comprises: purifying a(meth)acrylic acid mixture by a combination of at least two differenttypes of dynamic crystallization processes, and wherein the dynamiccrystallization processes used are a suspension crystallization and alayer crystallization.
 8. A process as claimed in claim 7, wherein thesuspension crystallization process is carried Out continuously.
 9. Aprocess as claimed in claim 7, wherein the layer crystallization processis carried out batchwise.
 10. A process as claimed in claim 7, whereinthe overall crystallization process is carried out using at least onestripping stage and at least one purification stage.
 11. A process asclaimed in claim 7, wherein the suspension crystallization process iscarried out below the layer crystallization.
 12. A process as claimed inclaim 7, wherein the overall crystallization process is carried outusing at least one stripping stage and at least one purification stage,wherein all stripping stages are carried out as a suspensioncrystallization and all purification stages as a layer crystallization.13. A process as claimed in claim 7, wherein the layer crystallizationis carried out as a falling-film layer crystallization.
 14. A process asclaimed in claim 7, wherein a starting mixture comprises from 75 to99.9% by weight, based on 100% by weight of starting mixture, of(meth)acrylic acid.
 15. A process as claimed in claim 7, wherein saidcombination of at least two different types of dynamic crystallizationprocesses is combined with at least one method selected from staticcrystallization and a distillation.
 16. A process for purifying(meth)acrylic acid by crystallization, which comprises: purifying a(meth)acrylic acid mixture by a combination of at least two differenttypes of dynamic crystallization processes, wherein a first dynamiccrystallization process is conducted in a dynamic crystallization deviceselected from the group consisting of a suspension crystallizationdevice, a falling-film layer crystallizer, a layer crystallizer having aflow through tube and a layer crystallizer having moving coolingsurfaces and a second dynamic crystallization process is conducted withone of said devices that is not selected as the device of the firstdynamic crystallization process.
 17. A process as claimed in claim 16,wherein the overall crystallization process occurs by employingfractional crystallization.
 18. A process as claimed in claim 16,wherein the purifying occurs by employing two different dynamiccrystallization processes.
 19. A process as claimed in claim 16, whereinall stripping stages are carried out as a suspension crystallization andall purification stages as a layer crystallization.
 20. A process asclaimed in claim 16, wherein said combination of at least two differentdynamic types of crystallization processes is combined with at least onemethod selected from static crystallization and a distillation.
 21. Aprocess as claimed in claims 16, wherein the overall crystallizationprocess is carried out using at least one stripping stage and at leastone purification stage.